If alien creatures exist elsewhere in our solar system, they're most likely to be found on Europa, one of 16 moons orbiting Jupiter.
There is strong evidence that beneath Europa's frozen exterior of ice lies an ocean of liquid water -- one of the essential ingredients for all living organisms.
Many scientists believe that this vast subterranean sea could host living microorganisms similar in size and complexity to bacteria found on Earth. Others question whether a frozen moon with a surface temperature of 260 o F (170o C) can produce sources of energy useful for the basic chemical reactions necessary for life.
But a new report in the Jan. 27 issue of the journal Nature concludes that Europa does indeed contain plenty of biological fuels, thanks to billions of charged particles that constantly rain down from neighboring Jupiter.
This relentless bombarbment of radiation "should produce organic and oxidant molecules sufficient to fuel a substantial Europan biosphere," writes Christopher Chyba, associate professor (research) of geological and environmental sciences.
On Earth, all organisms use carbon as a basic building block of life to construct everything from cells to DNA. Many organisms obtain their energy from carbon-based molecules like sugar, and some form of energy is required to free the carbon atoms from their chemical bonds.
Plants and algae use energy from sunlight to produce their own organic molecules out of carbon dioxide gas taken from the atmosphere or the ocean. The process is known as photosynthesis.
According to Chyba, sunlight would not provide enough energy to sustain life on Europa since its ocean appears to lie "beneath an ice layer too thick to permit photosynthesis."
A likelier source of energy, he concludes, may come from fast-moving, charged particles that pound Europa from the atmosphere of Jupiter. Jupiter has the strongest magnetic field of any planet," Chyba says, more than 10 times stronger than Earth's. When protons, electrons and other particles from space get trapped in Jupiter's magnetosphere, they are accelerated to extremely high velocities.
Europa's orbital path around Jupiter lies deep within this powerful magnetic field, so it receives a continuous barrage of electrified particles or ions.
According to Chyba, when these ions slam into the icy surface of the moon, chemical reactions are likely to occur, transforming frozen molecules of water and carbon dioxide into new organic compounds such as formaldehyde.
It turns out that one of the most common bacteria on Earth, Hyphomicrobium, survives on formaldehyde as its sole source of carbon, and Chyba believes that similar formaldehyde-feeding microbes could be alive and swimming in Europa's subsurface ocean.
In addition to creating organic fuels, radiation from Jupiter also may drive chemical reactions that produce oxidants -- molecules such as oxygen and hydrogen peroxide that can be used to burn formaldehyde and other carbon-based fuels.
But Chyba notes that the oxidant and organic molecules formed on Europa's frigid surface "are biologically relevant only if they reach the ocean."
The problem is that, if there is a liquid ocean on Europa, it's hidden beneath an ice sheet about 50 to100 miles (80 to 170 km) thick. So if extraterrestrial creatures are going to feast on formaldehyde, there has to be a way to get that compound through the dense layer of ice and into the liquid sea below.
Recent photographs taken by NASA's Galileo spacecraft reveal evidence of sudden melt-throughs in the ice that could allow oceanic microbes to come into quick contact with oxidants and organic food sources. The result could be a dramatic increase in population similar to "microbial blooms" that periodically occur in the Earth's oceans. Chyba points out that Europa's surface ice appears to get naturally recycled into the ocean every 10 million years -- a process that would allow a very gradual delivery of life-giving molecules to any submerged organisms. And just how many microbes might exist in Europa's sea? Chyba's conservative estimate: one per cubic centimeter -- a far cry from the hundreds of thousands of organisms that occupy each cubic centimeter of water on Earth.
Could life on our planet have its origins on Europa? Probably not, according to Chyba.
"Europa is as old as our solar system," he says, "but it's probably too far, too deep inside Jupiter's gravity well to have inoculated Earth with life-bearing debris."
Chyba emphasizes that all theories about life on Europa hinge on proof that a liquid body of water actually exists between the moon's surface and its rocky core.
"The point is to go there and find out," Chyba says, noting that in three years NASA plans to launch the Europa Orbiter satellite that will use radar to detect the presence of large bodies of subsurface water. The Orbiter should reach Europa in 2008, and NASA hopes to follow that with a remote landing.
"We'll know in the next 10 years if there's an ocean," Chyba predicts. "If there is, Europa will be the site of a series of new space missions."
As a student, Chyba's interest in extraterrestrial life led him to the Cornell University laboratory of famed astronomer Carl Sagan, a long-time advocate of planetary exploration. Chyba received his Ph.D. in astronomy under Sagan's guidance in 1985. Today, in addition to his post on the Stanford faculty, Chyba holds the Carl Sagan Chair for the Study of Life in the Universe at the SETI Institute in Mountain View, Calif. "SETI" is the acronym for the Search for Extraterrestrial Intelligence.
From 1993 to 1995, Chyba served as a White House adviser on national security. Beginning Feb. 1, he will become co-director of the Stanford Center for International Security and Cooperation, an organization dedicated to finding innovative solutions to worldwide security problems such as arms control and ethnic conflict.
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